1. Field of the Invention
This invention relates to a press-clamping terminal for being press-fastened at its conductor-clamping portion on an outer periphery of a conductor part of a wire so as to be electrically connected to the conductor part, and more particularly to an improvement in which an electrically-connected condition with a stable and small contact resistance can be maintained for a long period of time regardless of whether or not a passivation layer is formed on a surface of the conductor part of the wire.
2. Related Art
There are known various forms of connection of wires, and examples of such wire connection forms include a connection form using a press-clamping terminal, a connection form using a press-contacting terminal, a connection form in which a wire conductor is joined directly to a mating conductor by welding (spot welding or ultrasonic welding), and a connection form using soldering.
A press-clamping terminal is the type of connection terminal which is electrically connected to a conductor part of a wire by press-fastening a conductor-clamping portion thereof on an outer periphery of the conductor part. By changing the form of a distal end portion of such a terminal, various types, such as a screw-fastening type and a male-female fitting connection type, have been developed.
As compared with other wire connection forms, a wire connection form, using a press-clamping terminal, has advantages that the mechanical strength of connection to a wire can be easily secured and that an on-site operation can be carried out easily since only a compact press-fastening tool is required for connecting the press-clamping terminal to the wire. Therefore, this wire connection form still has a high utility value even at the present time.
By the way, electronic devices for being driven by a very small current/voltage have been extensively used in recent electric equipments, electronic equipments and so on, and therefore there is a possibility that a slight current fluctuation or a slight voltage fluctuation due to variations in contact resistance at a wire-connected portion causes a malfunction of a circuit.
Therefore, it has become important to maintain an electrically-connected condition with a stable and small contact resistance at the wire-connected portion over a long period of time.
Under this background, connection forms, shown respectively in FIGS. 4 and 5, have been proposed as a technique for reducing a contact resistance at the wire-connected portion.
In the connection form shown in FIG. 4, when a conductor-clamping portion 3 of a press-clamping terminal is to be press-fastened around a plurality of conductors (wire elements) 1, powder 4 of metal, softer than a material of which each conductor 1 is made, is beforehand coated onto an outer periphery of each of the conductors 1, and when the press-fastening operation is finished, interstices (or gaps) between the conductors 1 are filled with the metal powder 4, thereby increasing the electrical contact area so as to reduce the contact resistance and also to make the contact resistance stable for a long period of time (see, for example, JP-A-8-321330).
In the connection form shown in FIG. 5, when a conductor-clamping portion of a press-clamping terminal is to be press-fastened around a plurality of conductors (wire elements) 6, electrically-conductive powder 7, harder than a material of which each conductor 6 is made, is beforehand coated onto an outer periphery of each of the conductors 6, and when the press-fastening operation is finished, the electrically-conductive particles 7 pierce passivation layers (oxide layers) formed on the surfaces of the conductors 6, thereby preventing a contact resistance between the conductors 6, as well as a contact resistance between the conductors 6 and the conductor-clamping portion, from being increased by the intervening passivation layers 9 (see, for example, JP-A-8-321331).
Both of the countermeasures, proposed respectively in the above Patent Publications JP-A-8-321330 and JP-A-8-321331, have a problem that much time and labor are required for coating the metal powder 4 or the electrically-conductive powder 7 onto the wire conductors, so that the efficiency of the press-clamping connection operation is lowered.
And besides, both of the countermeasures, proposed respectively in the above Patent Publications JP-A-8-321330 and JP-A-8-321331, have another problem that it is not easy to coat the metal powder 4 or the electrically-conductive powder 7 uniformly onto the surfaces of the plurality of conductors, and the effect is liable to be varied by the uneven coating.
Furthermore, when a wire whose conductors are made of an aluminum base material or an iron-nickel alloy is exposed to the air, a passivation layer (oxide layer) is liable to be formed on the surfaces of the conductors. This passivation layer is harder than the conductor material, and is lower in electrical conductivity than the conductor material.
In the case of the above JP-A-8-321330, there has been encountered a problem that the metal powder 4 can not pierce the passivation layer, so that the increase of the contact resistance due to the intervening passivation layers can not be overcome.
On the other hand, in the case of the above JP-A-8-321331, it is quite possible that the electrically-conductive powder 7 pierces the passivation layers 9 if the electrically-conductive powder 7 is harder than the passivation layers 9. However, when the electrically-conductive powder 7, coated on the conductors 6, still has fluidity, the coated electrically-conductive powder 7 is caused to escape or flow into the interstices between the conductors 6 by a pressing force applied during the press-fastening operation, and only a very small part of the electrically-conductive powder 7 effectively serves to pierce the passivation layers 9, and this has resulted in a problem that it is difficult to obtain a good contact condition over a wide area of the inner surface of the conductor-clamping portion of the press-clamping terminal.